Hmm, it's hard to imagine how a person would fully appreciate Part III who had never seen Parts I or II. Michael's sin and guilt and search for repentance just wouldn't have the impact it you had not seen his crimes graphically displayed in the prior ones.
I actually agree with many of the criticisms of the movie's discontinuity—not only Michael's appearance is disjointed (as is Kay's); but Michael also seems to have had a personality transplant. But unlike many critics, I really “got” Sophia Coppola as a New York born-and-raised Italian-American princess. Her looks seemed to have been incomprehensible to audiences expecting the typical Hollywood starlet look, but are quite representative of authetic Greek-Sicilian bloodlines. I found it touching that her cousin and lover played by Andy Garcia took her back to New York City's Little Italy neighborhood to show her her “roots.”
Eli Wallach as Don Altobello was an amazing performance, including his command of Sicilian dialect (he is Jewish). And the scenes of Kay and Michael's son in I Vespri Siciliani was a great treat for opera lovers.
Sophia Coppola is actually a wonderful director herself—I particularly enjoyed two of her films, Marie Antoinette with Kirsten Dunst and Jason Schwartzman, and Lost in Translation with Bill Murray (in a serious role) and Scarlett Johansson. Sophia is currently directing a bio pic about Priscilla Presley.
I've watched GF1 & GF2. Never GF3. But, I read criticism of the casting of Sofia in her role. So, I watched a few scenes from the movie, and I agree with your take on it: She does look more authentic for that part. Some articles say Wynona Ryder bowed out, so Sofia stepped into the role. Hard to imagine Wynona Ryder in a Godfather movie.